1. Field of the Invention
The present invention relates to a charge transfer device, a process for manufacturing the device, and a method of driving the device.
2. Description of the Prior Art
A solid-state image sensor represented by a charge-coupled device (hereinafter "CCD") is excellent in low-noise characteristics and other properties, and is hence put in practical use intensively.
Referring first to the drawings, the structure and driving method of the charge transfer device used in the conventional solid-state image sensor is described below.
In FIGS. 15 to 18, the structure and driving method of the so-called four-phase driven charge transfer device is shown as a conventional example of a charge transfer device.
FIG. 15 describes the structure of the device. Numeral 1 denotes a p-type semiconductor substrate, 2 is an n.sup.- diffusion layer to be used as the channel portion of a so-called buried type channel CCD, 3 to 6 are transfer electrodes, 7 is an insulation layer of SiO.sub.2 or the like, and 8 to 11 are voltage application terminals to each transfer electrode.
FIG. 16 shows driving voltage waveforms, in which .phi.1 to .phi.4 are voltage waveforms to be applied to voltage application terminals 8 to 11, respectively. In the driving waveforms, the state of the voltage higher than zero volts is called H (high) state, and the state of the voltage lower than zero volts is called L (low) state.
FIG. 17 depicts the potential distribution of the channel portion 2 at time t1 to t5.
Regions A2 to F2 correspond to regions A1 to F1 shown in FIG. 15, and numerals 16, 17 are transfer charges.
As shown in FIG. 16, at t=t1, a high voltage (hereinafter indicated as "H") is applied to the voltage application terminals 8, 9. At the voltage application terminals 10, 11, a low voltage ("L") is applied. A voltage of H level is applied to the transfer electrodes 3, 4, and L level to the transfer electrodes 5, 6.
As shown in FIG. 17, the potential of the transfer channel corresponding to the electrode of H level becomes high (hereinafter called "potential well"). In this potential well, the electrons of the signal charges 16, 17 are stored.
Next, at t=t2, the voltage application terminal 10, that is, the transfer electrode 5 changes from L to H. As a result, the signal charges 16, 17 are stored in the potential well of the channel coupled to the transfer electrodes 3, 4.
At t=t3, the voltage application terminal 8, that is, the transfer electrode 3 is changed from H to L. Consequently, the potential well formed beneath the transfer electrode 3 is eliminated. The charges staying there move to the potential well beneath the transfer electrodes 4, 5.
By this sequence of operations, the charges 16, 17 are moved by the distance of one transfer electrode.
Thereafter, repeating the same operation, the charges are sequentially transferred. Thus, in the charge transfer device, the signal charges must be transferred to the next potential well exactly within a specified time.
FIG. 18 shows the transient state of moving from t=t4 to t=t5 in FIG. 17, in which numeral 21 denotes the potential distribution at t=t4, 24 is the signal charges at this time, 22 is the potential distribution at the transient time, 25 is the charges at this time, 23 is the potential distribution at t=t5, and 20 expresses the movement of charges.
As the voltage applied to the transfer electrode 4 changes from H to L, the channel portion coupled to the transfer electrode 4 changes from the state of potential 21 to the state of potential 23. At this time, the charges 24, which comprise a majority of the charges coupled to the transfer electrode 4, are moved to the potential well beneath the next transfer electrode 5 by the repulsive force between charges. However, slight charges 25 are left over. It is the force from the diffusion and fringe electric force that moves these charges 25 into the potential well beneath the next transfer electrode 5. By the end of the move of the charges 25, the transfer is terminated.
In the constitution of the conventional charge transfer device as stated above, the following defects were involved.
That is, in the conventional charge transfer device, since the length of the adjacent transfer electrodes is equal, charges are often left over, and the transfer efficiency is poor.
There is only one diffusion layer beneath the transfer electrodes, and when transferring the charges stored in the diffusion layer, they are left over.
Besides, because of only one diffusion layer beneath the transfer electrodes, the channel potential difference is needed as much as the drive pulse voltage, and the control performance is poor.
Since only one diffusion layer is disposed beneath the transfer electrodes, the effective gate length cannot be shortened in order to heighten the transfer speed.
In the conventional charge transfer device, since the length of the adjacent transfer electrodes is equal, the transfer speed of the charge transfer device cannot be quickened.
Thus, in the conventional charge transfer device, although all charges must be transferred within a limited time, some charges are sometimes left over, and 100% transfer efficiency is not achieved. The remaining charges appear as a blurring of the image in the image sensor, and the picture quality is degraded.
In the light of the above demerits, it is hence a primary object of the invention to present a charge transfer device of high transfer efficiency with less left-over charges, a process for manufacturing the device, and a method of driving the device.
It is another object of the invention to present a charge transfer device of which channel potential difference is about half the drive pulse voltage, a method for manufacturing the device, and a method of driving the device.
It is another object of the invention to present a charge transfer device shortened in the effective gate length so as to increase the transfer speed, a method for manufacturing the device, and a method of driving the device.
It is a further object of the invention to present a charge transfer device capable of enhancing the picture quality by transferring almost all charges within a limited time, while preventing blurring of image on an image sensor, a method for manufacturing the device, and a method of driving the device.